The need of being able to amplify the power level of a signal to a given reference level is found in several technical applications, said reference level conceivably varying with time.
A power amplifier in a mobile radio transmitter is an example of one such technical amplification. The base station receiving the signal from the mobile radio transmitter will order the transmitter to transmit with different powers, depending on where the transmitter is located in relation to the base station. In the case of a mobile radio transmitter, which is normally battery powered, it is also important to ensure that the signal is amplified to each given reference level with the highest possible efficiency--with the intention that the amplifier will amplify the signal to the desired level with the lowest possible total power and current consumption. The smaller the power consumption, the less heat developed, while the smaller the current consumption, the longer the mobile radio transmitter can be used without needing to recharge the battery.
Efficiency can be measured in several ways. A typical measurement of efficiency is the so-called power added efficiency, wherewith the power difference between the signal whose power is to be amplified and the power-amplified signal is related to the total DC-power delivered to the power amplifier. This method of measuring efficiency results in high efficiency even at low current consumptions. By efficiency is meant power added efficiency in the following text, unless otherwise stated.
The efficiency of a power amplifier is normally improved by providing special matching circuits at the input and output of the power amplifier and between the amplifying stages, in order to match input and output impedances between the different stages, so that as much power as possible will be transferred from one stage to the next stage. The matching criterion is that the output impedance from one stage shall correspond to the complex conjugate of the input impedance to the following stage.
One method of obtaining regulated power amplification of a power amplifier is to dimension the power amplifier so that it will have good efficiency for the highest value of the reference level and then measuring the power output and, on the basis of this measurement, attenuating the input signal or controlling either the supply voltages or the bias voltages of the transistors in the power amplifier, so that the output power will correspond to the reference level.
One drawback with this method is that one or more of the amplifying stages will operate ineffectively for reference-level values that are lower than the highest reference-level value. The efficiency will thus be unnecessarily low for these reference-level values.
Patent Specification SE-A 8900860 teaches a power amplifying circuit which amplifies power to a predetermined number of reference-level values, where measurements and steps are taken to obtain high efficiency for all reference-level values. The circuitry includes output power measuring means. The supply voltages to some of the transistors included in the amplifier are adjusted until the measured output power coincides with the reference value in question. The bias voltages for these transistors are set to predetermined values for each reference level, said values being chosen so that the amplifier will nominally have a higher efficiency. The construction can thus be said to provide closed adjustment or regulation of the output power by adjusting the supply voltages and setting the bias voltages in relation to the reference level in question, therewith improving efficiency.
This construction is encumbered with a number of drawbacks, however. Firstly, the bias-voltage setting must be sensitive to parameter variations of the transistors in the power amplifier, for instance knee-voltage variations. This means that the same bias-voltage settings cannot be used reliably for different circuits, even when the circuits have the same construction and the same nominal parameter and component values. Consequently, in order to reliably obtain a good efficiency, it is necessary to determine suitable bias-voltage settings for each circuit per se which can, of course, be very time-consuming, especially when the circuits shall be mass produced. Secondly, the bias voltages are set to the same value for all transistors involved in the control, which is generally not optimal from the aspect of manufacture. Thirdly, the changes in supply voltages and bias voltages result in changes in the input and output impedances of the amplifying stages of the power amplifier. Impedance matching between the amplifying stages and the power amplifier input and output respectively will not therefore always be satisfactory, which results in unnecessarily low efficiency.